This invention relates generally to radioactivity well logging, and particularly, to methods and apparatus for deriving the fluid-filled porosity of the formations surrounding an earth borehole.
It is well known in the prior art to utilize a pulsed neutron source and a pair of spaced radiation detectors to provide an indication of the formation porosity. With such prior art devices, the near-spaced detector is gated on during some interval between the neutron burst and the far-spaced detector is similarly gated on during some gating interval between bursts. The signals from the first detector are then combined with the signals from the second detector to result in a ratio which is indicative of the formation porosity.
For example, in U.S. Pat. No. 3,379,882 to Arthur H. Youmans which issued on Apr. 23, 1968, and which is assigned to the assignee of the present application, it is disclosed with respect to FIG. 13 of that patent that the ratio is achieved by combining signals from the two detectors in a ratio circuit and specifically that the gating intervals should be identical.
Furthermore, in U.S. Pat. No. 3,621,255 to Robert J. Schwartz which issued on Nov. 16, 1971, there is disclosed a concept much like the aforementioned Youmans patent which is said to provide an indication of the formation porosity by producing signals from each of the detectors which are combined in ratio circuitry to provide a signal which is related to the porosity of the formation. The circuitry described in the Schwartz patent speaks of automatically adjusting the gates in the two detectors in accordance with the formation characteristics being logged, but does not particularly indicate how the gates associated with one detector would relate in time to the gates associated with the other detector.
Furthermore, in U.S. Pat. application Ser. No. 806,050, filed June 13, 1977, in the name of Eric C. Hopkinson, and assigned to the assignee of the present application, there is disclosed circuitry such that the radiation detectors are gated differently from each other to provide an indication of formation porosity which is substantially independent of the formation salinity. In the preferred embodiment of that application, the electrical signals indicative of radiation detected by the long-spaced detector are gated for almost the entire interval between neutron pulses and the short-spaced signals are gated for a significantly smaller time interval which commences soon after the termination of a given neutron burst. The signals from the two detectors are combined in a ratio circuit for determination of porosity.
In short, the prior art system for determining porosity from dual spaced detectors teaches that the intensity observed by differently spaced detectors can be used to indicate the porosity in the ratio system.
Although the prior art method and apparatus have enjoyed a fair degree of success in determining the fluid-filled porosity of earth formations, there nonetheless exists a need for a system which is less affected by salinity and other borehole effects.
Furthermore, in U.S. Pat. No. 3,868,505 to L. A. Jacobsen et al, issued on Feb. 25, 1975, there is disclosed the use of dual spaced detectors from which a long-spaced and a short-spaced Sigma is derived, but as an indication of the quality of the Sigma from the short-spaced detector because of a so-called diffusion correction. In fact, however, the system in accordance with that patent needs an indication of porosity to make an accurate diffusion correction. Such a system can be classified as a diffusion-corrected measurement of the macroscopic absorption cross section.
It is therefore the primary object of the present invention to provide new and improved methods and apparatus for determining the fluid-filled porosity of earth formations surrounding a borehole;
It is also an object of the present invention to provide new and improved methods and apparatus for simultaneously measuring the porosity and the macroscopic absorption cross section of a formation.
The objects of the invention are achieved, generally, by methods and apparatus which utilize a source of pulsed neutrons and a pair of radiation detectors from which a pair of macroscopic absorption cross section measurements are derived and the difference between such measurements is utilized as being functionally related to the fluid-filled porosity of the earth formations surrounding the borehole.